Gate spacer and inner spacer formation for nanosheet transistors having relatively small space between gates

ABSTRACT

Embodiments of the invention are directed to method of fabricating a semiconductor device. A non-limiting embodiment of the method includes performing fabrication operations to form a nanosheet field effect transistor (FET) device on a substrate, wherein the fabrication operations include forming gate spacers along a gate region of the nanosheet FET device, wherein each of the gate spacers comprises an upper segment and a lower segment.

DOMESTIC PRIORITY

This application is a continuation of U.S. application Ser. No.15/944,322, filed Apr. 3, 2018, the contents of which are incorporatedby reference herein in its entirety.

BACKGROUND

The present invention relates in general to fabrication methods andresulting structures for semiconductor devices. More specifically, thepresent invention relates to fabrication methods and resultingstructures for forming the offset gate spacers (or outer spacers) andthe inner spacers in nanosheet transistors having relatively smallspaces (e.g., below about 10 nm) between the gates of adjacent nanosheettransistors during intermediate fabrication steps.

In contemporary semiconductor device fabrication processes, a largenumber of semiconductor devices, such as silicon channel n-type fieldeffect transistors (nFETs) and silicon germanium channel p-type fieldeffect transistors (pFETs), are fabricated on a single wafer. Non-planartransistor device architectures, such as nanosheet (or nanowire)transistors, can provide increased device density and increasedperformance over planar transistors. Nanosheet transistors, in contrastto conventional planar FETs, include a gate stack that wraps around thefull perimeter of multiple nanosheet channel regions for improvedcontrol of channel current flow. Nanosheet transistor configurationsenable fuller depletion in the nanosheet channel regions and reduceshort-channel effects.

SUMMARY

Embodiments of the invention are directed to method of fabricating asemiconductor device. A non-limiting embodiment of the method includesperforming fabrication operations to form a nanosheet field effecttransistor (FET) device on a substrate, wherein the fabricationoperations include forming gate spacers along a gate region of thenanosheet FET device, wherein each of the gate spacers comprises anupper segment and a lower segment.

Embodiments of the invention are directed to a method of fabricating asemiconductor device. A non-limiting embodiment of the method includesperforming fabrication operations to form nanosheet FET devices on asubstrate. The fabrication operations include forming gate spacers alonga gate region of the nanosheet FET device. Inner spacers are formedbetween channel nanosheets of the nanosheet FET device. Each of the gatespacers includes an upper segment and a lower segment. The upper segmentincludes a second type of material, the lower segment includes a firsttype of material, and the inner spacers include the first type ofmaterial.

Embodiments of the invention are directed to a nanosheet FET device. Anon-limiting example of the nanosheet field FET device includes gatespacers formed along a gate region of the nanosheet FET device, whereineach of the gate spacers includes an upper segment and a lower segment.

Additional features and advantages are realized through techniquesdescribed herein. Other embodiments and aspects are described in detailherein. For a better understanding, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as embodiments is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments are apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a cross-sectional view of a section of a substrate/waferhaving nanosheet transistors formed thereon, wherein the nanosheettransistors include offset gate spacers (or outer spacers) and innerspacers according to embodiments of the invention;

FIGS. 2-15 depict cross-sectional views of a section of asubstrate/wafer after various fabrication operations to form thenanosheet transistors shown in FIG. 1 according to embodiments of theinvention, in which:

FIG. 2 depicts a cross-sectional view of a semiconductor structure afterinitial fabrication stages according to embodiments of the invention;

FIG. 3A depicts a cross-sectional, across-the-fin view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 3B depicts a cross-sectional, across-the-gate view of thesemiconductor structure shown in FIG. 3A, wherein the across-the-gateview results from rotating the across-the-fin view 90 degreescounterclockwise into the page;

FIG. 4 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 5 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 6 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 7 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 8 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 9 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 10 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 11 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 12 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 13 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention;

FIG. 14 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention; and

FIG. 15 depicts a cross-sectional, across-the gate view of thesemiconductor structure after fabrication stages according toembodiments of the invention.

DETAILED DESCRIPTION

It is understood in advance that although this invention includes adetailed description of exemplary gate-all-around (GAA) nano sheet FETarchitectures having silicon (Si) channel nanosheets and SiGesacrificial nanosheets, embodiments of the invention are not limited tothe particular FET architectures or materials described in thisspecification. Rather, embodiments of the present invention are capableof being implemented in conjunction with any other type ofnanosheet/nanowire FET architecture or materials now known or laterdeveloped. In this detailed description and in the claims, the termsnanosheet and nanowire are treated as being synonymous.

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to a description of technologies that are more specificallyrelevant to the present invention, transistors are semiconductor devicescommonly found in a wide variety of ICs. A transistor is essentially aswitch. When a voltage is applied to a gate of the transistor that isgreater than a threshold voltage, the switch is turned on, and currentflows through the transistor. When the voltage at the gate is less thanthe threshold voltage, the switch is off, and current does not flowthrough the transistor.

Typical semiconductor devices are formed using active regions of awafer. The active regions are defined by isolation regions used toseparate and electrically isolate adjacent semiconductor devices. Forexample, in an IC having a plurality of metal oxide semiconductor fieldeffect transistors (MOSFETs), each MOSFET has a source and a drain thatare formed in an active region of a semiconductor layer by implantingn-type or p-type impurities in the layer of semiconductor material.Disposed between the source and the drain is a channel (or body) region.Disposed above the body region is a gate electrode. The gate electrodeand the body are spaced apart by a gate dielectric layer. Complementarymetal oxide semiconductor (CMOS) is a technology that uses complementaryand symmetrical pairs of p-type and n-type MOSFETs to implement logicfunctions. The channel region connects the source and the drain, andelectrical current flows through the channel region from the source tothe drain. The electrical current flow is induced in the channel regionby a voltage applied at the gate electrode.

The wafer footprint of an FET is related to the electrical conductivityof the channel material. If the channel material has a relatively highconductivity, the FET can be made with a correspondingly smaller waferfootprint. A known method of increasing channel conductivity anddecreasing FET size is to form the channel as a nanostructure. Forexample, a so-called gate-all-around (GAA) nanosheet FET is a knownarchitecture for providing a relatively small FET footprint by formingthe channel region as a series of nano sheets. In a known GAAconfiguration, a nanosheet-based FET includes a source region, a drainregion and stacked nanosheet channels between the source and drainregions. A gate surrounds the stacked nanosheet channels and regulateselectron flow through the nanosheet channels between the source anddrain regions. GAA nanosheet FETs are fabricated by forming alternatinglayers of channel nanosheets and sacrificial nanosheets. The sacrificialnanosheets are released from the channel nanosheets before the FETdevice is finalized. For n-type FETs, the channel nanosheets aretypically silicon (Si) and the sacrificial nanosheets are typicallysilicon germanium (SiGe). For p-type FETs, the channel nanosheets aretypically SiGe and the sacrificial nanosheets are typically Si. In someimplementations, the channel nanosheet of a p-FET can be SiGe or Si, andthe sacrificial nanosheets can be Si or SiGe. Forming the GAA nanosheetsfrom alternating layers of channel nanosheets formed from a first typeof semiconductor material (e.g., Si for n-type FETs, and SiGe for p-typeFETs) and sacrificial nanosheets formed from a second type ofsemiconductor material (e.g., SiGe for n-type FETs, and Si for p-typeFETs) provides superior channel electrostatics control, which isnecessary for continuously scaling gate lengths down to seven (7)nanometer CMOS technology and below. The use of multiple layered SiGe/Sisacrificial/channel nanosheets (or Si/SiGe sacrificial/channelnanosheets) to form the channel regions in GAA FET semiconductor devicesprovides desirable device characteristics, including the introduction ofstrain at the interface between SiGe and Si.

Although nanosheet channel FET architectures provide increased devicedensity over planar FET architectures, there are still challenges whenattempting to fabricate nanosheet channel FETs having increasinglysmaller gate pitch. For example, known inner spacer formation techniquesmay or may not work when the gate pitch becomes smaller and smaller. Insome nanosheet transistor architectures, inner spacers are providedbetween adjacent ones of the stacked channel nanosheets at end regionsthereof. The inner spacers protect the channel nanosheets from thesource and drain regions, and also provide electric isolation andsupport between the channels. As the gate pitch for nanosheettransistors continues to decrease, the space between the gates ofadjacent devices on the chip is very small (e.g., 6 nm for current 48cpp (contacted poly/gate pitch), 2 nm for 44 cpp, etc.), which makes itchallenging to apply known inner spacer fabrication techniques that weredeveloped for gate pitches below about 44 CPP.

Turning now to an overview of aspects of the invention, embodiments ofthe invention provides fabrication methods and resulting structures forforming the offset gate spacers (or outer spacers) and the inner spacersin nanosheet transistors having relatively small spaces between thegates of adjacent nanosheet transistors (e.g., below about 7 nm). Inembodiments of the invention, a dummy spacer is utilized to, in effect,“bookmark” or “reserve” the place for the outer spacers. In downstreamfabrication operations, the dummy (or placeholder) spacer is replacedwith two-segment outer spacers and inner spacers. In embodiments of theinvention, the lower segment of the two-segment outer spacers is thesame material as the inner spacers. The likelihood of gate pinch-off isreduced and spacer structural integrity is increased by utilizing aself-aligned spacer etch to remove the excess inner spacer materialcovering the Si channel nanosheet with selectivity to the top-segment ofthe two-segment outer spacers.

Turning now to a more detailed description of fabrication operationsaccording to aspects of the invention, FIG. 1 depicts a cross-sectionalview of a section 100 of a substrate/wafer 102 having nanosheettransistors 140, 140A formed thereon, wherein each of the nanosheettransistors 140, 140A includes offset gate spacers (or outer spacers)1202 (best shown in FIG. 13) and inner spacers 902C according toembodiments of the invention. In the illustrated embodiments, thesubstrate 102 includes a bulk silicon material. Alternatively, thesubstrate 102 can be implemented in a semiconductor-on-insulator (SOI)configuration. For ease of illustration and description, two nanosheettransistors 140, 140A are shown on the substrate 102. However, it shouldbe understood that the teachings in this detailed description can beapplied to the fabrication of a substrate/wafer having any number ofnanosheet transistors formed thereon. The nanosheet transistors 140,140A are substantially identical, and so reference numbers for elementsof one of the nanosheet transistors 140, 140A applies equally to thecorresponding element on the other nanosheet transistor.

As shown in FIG. 1, and in accordance with aspects of the invention,each nanosheet transistor 140, 140A includes nanosheet channels, 122B,124B, 126B; source/drain (S/D) regions 1310, 1312 at end regions of thenanosheet channels, 122B, 124B, 126B; metal gate structures 1406, workfunction metal (WFM) regions 1404, and gate dielectric (e.g., high-k)layers 1402 that extend over and around the nanosheet channels, 122B,124B, 126B in a GAA configuration; offset gate spacers 1202 (best shownin FIG. 13) formed from upper segments 1102A and lower segments 902B;and inner spacers 902C, configured and arranged as shown. An interlayerdielectric (ILD) region 1302 is formed over the substrate 102 and thenanosheet transistors 140, 140A.

In embodiments of the invention, and as described in greater detail inconnection with the fabrication operations shown in FIGS. 2-15,fabrication methods and resulting structures are described for formingthe offset gate spacers (or outer spacers) 1202 and the inner spacers902C in nanosheet transistors 140, 140A, wherein relatively small spaceis provided between upper regions of the metal gates 1406 of adjacentnanosheet transistors (e.g., below about 10 nm). In embodiments of theinvention, dummy spacers 502A (shown in FIG. 7) are utilized to, ineffect, “bookmark” or “reserve” the place for the offset gate spacers1202. In downstream fabrication operations, the dummy spacers arereplaced with two-segment outer spacers 1202 and inner spacers 902C. Inembodiments of the invention, the upper segment 1102 a of thetwo-segment offset gate spacer 1202 is a different material than thelower segment 902B of the two-segment offset gate spacer 1202 and thelower segment 902B is the same material as the inner spacer 902C. Thelikelihood of gate pinch-off is reduced and spacer structural integrityis increased by utilizing a self-aligned spacer etch to remove theexcess inner spacer material covering the Si channel nanosheets 122B,124B, 126B with selectivity to the upper segment 1102A. The selectivityof the self-aligned spacer etch is facilitated because the upper segment1102A is different material than the lower segment 902B and the innerspacer 902C.

FIGS. 2-15 depict cross-sectional views of section 100 of thesubstrate/wafer 102 (shown in FIG. 1) after various fabricationoperations to form the nanosheet transistors 140, 140A on the samesubstrate 102 in accordance with embodiments of the invention. As shownin FIG. 2, after initial stage an alternating series of SiGe sacrificialnanosheet layers 112, 114, 116, 118 and Si nanosheet layers 122, 124,126 are formed in a stack on the Si substrate 102. In some embodimentsof the invention, the SiGe nanosheet layers 112, 114, 116, 118 can bewithin a range from SiGe 20% to SiGe 40%. The notations “SiGe 20%” and“SiGe 40%” are used to indicate that 20% of the SiGe material is Ge, orthat 40% of the SiGe material is Ge, respectively.

In embodiments of the invention, the alternating nanosheet layers 112,122, 114, 124, 116, 126, 118 depicted in FIG. 2 can be formed byepitaxially growing one layer and then the next until the desired numberand desired thicknesses of the nanosheet layers are achieved. Epitaxialmaterials can be grown from gaseous or liquid precursors. Epitaxialmaterials can be grown using vapor-phase epitaxy (VPE), molecular-beamepitaxy (MBE), liquid-phase epitaxy (LPE), or other suitable process.Epitaxial silicon, silicon germanium, and/or carbon doped silicon (Si:C)silicon can be doped during deposition (in-situ doped) by addingdopants, n-type dopants (e.g., phosphorus or arsenic) or p-type dopants(e.g., boron or gallium), depending on the type of transistor.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown” mean the growth of a semiconductor material (crystallinematerial) on a deposition surface of another semiconductor material(crystalline material), in which the semiconductor material being grown(crystalline overlayer) has substantially the same crystallinecharacteristics as the semiconductor material of the deposition surface(seed material). In an epitaxial deposition process, the chemicalreactants provided by the source gases are controlled and the systemparameters are set so that the depositing atoms arrive at the depositionsurface of the semiconductor substrate with sufficient energy to moveabout on the surface such that the depositing atoms orient themselves tothe crystal arrangement of the atoms of the deposition surface.Therefore, an epitaxially grown semiconductor material has substantiallythe same crystalline characteristics as the deposition surface on whichthe epitaxially grown material is formed. For example, an epitaxiallygrown semiconductor material deposited on a {100} orientated crystallinesurface will take on a {100} orientation. In some embodiments, epitaxialgrowth and/or deposition processes are selective to forming onsemiconductor surfaces, and generally do not deposit material on exposedsurfaces, such as silicon dioxide or silicon nitride surfaces.

In some embodiments, the gas source for the deposition of epitaxialsemiconductor material include a silicon containing gas source, agermanium containing gas source, or a combination thereof. For example,an epitaxial silicon layer can be deposited from a silicon gas sourcethat is selected from the group consisting of silane, disilane,trisilane, tetrasilane, hexachlorodisilane, tetrachlorosilane,dichlorosilane, trichlorosilane, methylsilane, dimethylsilane,ethylsilane, methyldisilane, dimethyldisilane, hexamethyldisilane andcombinations thereof. An epitaxial germanium layer can be deposited froma germanium gas source that is selected from the group consisting ofgermane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. While an epitaxial silicongermanium alloy layer can be formed utilizing a combination of such gassources. Carrier gases like hydrogen, nitrogen, helium and argon can beused.

FIG. 3A depicts a cross-sectional, across-the-fin view of thesection/portion 100A of the substrate 102 after fabrication stagesaccording to embodiments of the invention, and FIG. 3B depicts across-sectional, across-the-gate view of the section/portion 100A of thesubstrate 102 shown in FIG. 3A, wherein the across-the-gate view resultsfrom rotating the across-the-fin view 90 degrees counterclockwise intothe page. In FIGS. 3A and 3B, known semiconductor fabrication operationshave been used to etch or recess the alternating nanosheet layers 112,122, 114, 124, 116, 126, 118 shown in FIG. 2 to form multipleintermediate elongated columns 202. In an example fabrication process, apatterned hard mask (not shown) is deposited over the alternatingnanosheet layers 112, 122, 114, 124, 116, 126, 118. The pattern of thehard mask defines the footprints of the multiple intermediate elongatedcolumns 202 shown in FIGS. 3A and 3B. FIG. 3A is a view of theintermediate elongated columns 202 by a person standing in front thenarrowest sidewalls of the intermediate elongated columns 202 (i.e., inthe across-fin direction). FIG. 3B is a view of one of the intermediateelongated columns 202 by a person standing in front of the elongatedsidewall of one of the intermediate elongated columns 202 (i.e., in theacross-gate direction). An etch (e.g., an RIE) or a recess is applied toremove the portions of the alternating nanosheet layers that are notcovered by the patterned hard mask, thereby forming the intermediateelongated columns 202.

The remaining FIGS. 4-15 are the cross-sectional view, across-the-gateview used in FIG. 3B. In FIG. 4, known semiconductor fabricationoperations have been used to form a thin gate dielectric 302, anamorphous silicon (a-Si) region 303, a SiN hard mask layer 305, and aSiO₂ hard mask layer 307, configured and arranged as shown. Althoughonly one of the intermediate elongated fin-shaped columns 202 is shownin the two-dimensional (X-axis and Y-axis) cross-sectional diagram shownin FIG. 4, multiple additional intermediate elongated fin-shaped columns(not shown) are positioned behind the intermediate elongated fin-shapedcolumn 202 and extend into the page in the Y-axis direction. Theadditional intermediate elongated fin-shaped columns positioned behindthe intermediate elongated fin-shaped column 202 are substantially thesame as the intermediate elongated fin-shaped column 202. Similarly,although the a-Si region 303 is depicted in a cross-sectional,two-dimensional view in FIG. 4, in practice, the aSi region 303 isformed over and around the elongated sidewalls of each of theintermediate elongated fin-shaped columns 202.

In FIG. 5, known semiconductor fabrication operations have been used toform the dummy gates 304. For example, the dummy gates 304 can be formedby patterning the SiN hard mask layer 305 (shown in FIG. 4) and the SiO₂hard mask layer 307 (shown in FIG. 4) to form the hard masks 306, 308.An etching process (e.g., an RIE) is applied to the a-Si region 303(shown in FIG. 4) to form the dummy gates 304.

In FIGS. 6 and 7, known fabrication operations have been used to formthe dummy offset gate spacers 502A. For example, the dummy offsetspacers 502A (shown in FIG. 7) can be formed by a conformal deposition(shown in FIG. 6) of a dielectric material 502 (e.g., silicon oxide,silicon nitride, silicon oxynitride, SiBCN, SiOCN, SiOC, or any suitablecombination of those materials) to a thickness a1, followed by adirectional etch (e.g., RIE) to form the dummy offset gate spacers 502Ashown in FIG. 7. In embodiments of the invention, the dummy offset gatespacers 502A can be formed using a spacer pull down formation process.

Fabrication of the dummy offset gate spacers 502A result in exposedsurfaces of the gate dielectric 302 (shown in FIG. 6). In FIG. 7, anetch or a recess has been applied to the exposed surfaces of the gatedielectric 302 (shown in FIG. 6) and the intermediate elongated columns202 (shown in FIG. 6) to form the gate dielectric 302A and two adjacentsets of the final fin-shaped columns 202A. The etch/recess forms thegate dielectric 302A and the fin-shaped columns 202A byetching/recessing the portions of the intermediate elongated fin-shapedcolumns 202 and the gate dielectric 302 that are not covered by thedummy gates 304 and the dummy offset gate spacers 502A. Each of thefin-shaped columns 202A includes a stack of alternating SiGe/Sinanosheet regions 112A, 122A, 114A, 124A, 116A, 126A, 118A.

In FIG. 8, known semiconductor fabrication processes have been used toform pull back end region cavities 802 in the SiGe sacrificial nanosheetregions 114A, 116A, 118A (shown in FIG. 7) from underneath the dummyoffset gate spacers 502A using, for example, a hydrogen chloride (HCL)gas isotropic etch process, which etches SiGe without attacking Si. Theend region cavities 802 are formed to thicknesses that substantiallymatch the desired thickness of the to-be-formed inner spacers 902C(shown in FIG. 14). After formation of the end region cavities 802,sacrificial nanosheet central regions 112B, 114B, 116B, 118B remain.

In FIG. 9, known semiconductor fabrication processes have been used toremove the dummy offset spacers 502A. Portions of the gate dielectric302A have also been removed to form gate dielectric 302B. In accordancewith aspects of the invention, the dummy spacers 502A (shown in FIG. 7)were utilized to, in effect, “bookmark” or “reserve” the place for theoffset gate spacers 1202 (shown in FIG. 14). In downstream fabricationoperations, which are depicted in FIGS. 10-14, the dummy offset gatespacers 502A are replaced with two-segment outer spacers 1202 and innerspacers 902C. In embodiments of the invention, the upper segment 1102 aof the two-segment offset gate spacer 1202 is a different material thanthe lower segment 902B of the two-segment offset gate spacer 1202 andthe lower segment 902B is the same material as the inner spacer 902C.The likelihood of gate pinch-off is reduced and spacer structuralintegrity is increased by utilizing a self-aligned spacer etch (shown inFIG. 14) to remove the excess inner spacer material covering the Sichannel nanosheets 122B, 124B, 126B with selectivity to the uppersegment 1102A. The selectivity of the self-aligned spacer etch isfacilitated because the upper segment 1102A is different material thanthe lower segment 902B and the inner spacer 902C.

In FIG. 10, known semiconductor fabrication processes have been used todeposit a dielectric layer 902 formed from a first low-k material. Inembodiments of the invention, the first low-k material fills in the pullback end region cavities 802 (shown in FIG. 9). In embodiments of theinvention, the dielectric layer 902 is formed to a thickness a2. Inembodiments of the invention, a2 is substantially equal to or slightlysmaller than a1, wherein a1 is the thickness of the dummy spacers 502A(shown in FIG. 8) that were utilized to, in effect, “bookmark” or“reserve” the place for the offset gate spacers 1202 (shown in FIG. 14).Expressed as a percentage, a2 qualifies as being slightly smaller thana1 if a2 is less than a1 and within about 20% of a1.

In FIG. 11, known semiconductor fabrication processes have been used todeposit an organic planarization layer (OPL) 1002 and recess it to apredetermined level as shown. In embodiments of the invention, thepredetermined level of the OPL 1002 will be used in downstreamfabrication operations to define the interface between the upper segment1102A and the lower segment 902B of the two-component offset gatespacers 1202 (shown in FIG. 14). In embodiments of the invention, thepredetermined level is above the top nanosheet (i.e., sacrificialnanosheet 118B). In general, OPLs are used as etch masks for patterntransfers into inorganic substrates, to fill pre-existing features, andto planarize the substrate to allow for larger patterning processwindows.

In FIG. 12, known semiconductor fabrication processes have been used tostrip the exposed portions of the dielectric layer 902, leaving behinddielectric layer 902A. Also shown in FIG. 12, known semiconductorfabrication operations have been used to deposit a dielectric layer 1102formed from a second low-k material (e.g., LTO, SiOC, and the like). Inembodiments of the invention, the dielectric layer 1102 is formed toabout the thickness a2. In embodiments of the invention, a2 issubstantially equal to or slightly smaller than a1, wherein a1 is thethickness of the dummy spacers 502A (shown in FIG. 8) that were utilizedto, in effect, “bookmark” or “reserve” the place for the offset gatespacers 1202 (shown in FIG. 14). Expressed as a percentage, a2 qualifiesas being slightly smaller than a1 if a2 is less than a1 and within about20% of a1.

In FIG. 13, known semiconductor fabrication processes have been appliedto remove selected portions of the dielectric layer 1102 and the entireOPL 1002. In embodiments of the invention, the known semiconductorfabrication processes include multiple removal processes. In embodimentsof the invention, one of the removal processes is a directional etchselective to the second type of low-k material, which is tailored (e.g.,timed) to remove the portions of the dielectric layer 1102 that coverthe OPL region 1002 and the hard masks 308. After applying the directionetch to the dielectric layer 902, a dielectric layer 1102A remains, andtop surfaces of the OPL regions 1002 are exposed. In another of theremoval processes, the OPL region 1002 is removed using a removalprocess that is selective to the OPL material. In embodiments of theinvention, the OPL region 1002 can be removed by a dry removal processsuch as a reactive ion etch (RIE) or plasma ash.

In FIG. 14, in accordance with embodiments of the invention aself-aligned spacer etch has been applied to remove the excess portionsof the dielectric layer 902A (shown in FIG. 13) that are not covered bythe dielectric layer 1102A or the end regions of the channel nanosheets122A, 124A, 26A, thereby resulting in the formation of dielectric layers902B and inner spacers 902C. Forming the upper and lower segments(1102A, 902B) of the offset gate spacers 1202 from low-k materialsreduces the gate to S/D capacitance. Forming the inner spacers 902C fromlow-k material also reduces the gate to S/D capacitance. In embodimentsof the invention, the self-aligned spacer etch includes a directionaletch selective to the first type of low-k material. Because thedielectric layer 1102A is a different material (e.g., the second low-kmaterial) than the dielectric layer 902A (e.g., the first low-kmaterial) (shown in FIG. 13), the formation of dielectric layer 902B canbe defined by (or aligned by) the dielectric layer 1102A. Similarly,because the end regions of the nanosheet channels 122A, 124A, 126A are adifferent material (e.g., Si) than the dielectric layer 902A (e.g., thefirst low-k material) (shown in FIG. 13), the formation of inner spacers902C can be defined by (or aligned by) the end regions of the nanosheetchannels 122A, 124A, 126A. Accordingly, the thickness of the dielectriclayer 902B is determined by the thickness a2 of the dielectric layer1102A, and the thicknesses of the inner spacers 902C are determined bythe thicknesses of the end region cavities 802 (shown in FIG. 8). Inembodiments of the invention, the likelihood of gate pinch-off isreduced and spacer structural integrity is increased by utilizing theabove-described self-aligned spacer etch. In embodiments of theinvention, the self-aligned spacer etch is of sufficient duration toremove the dielectric layer 902A from end sidewalls of the channel nanosheets 122A, 124A, 126A.

In accordance with embodiments of the invention, the fabricationoperations depicted in FIGS. 13 and 14 result in the formation of thetwo-segmented offset gate spacers 1202 and inner spacers 902C. Thetwo-segmented offset gate spacers 1202 are formed from an upper segment1102A that includes a second type of low-k material, along with a lowersegment 902B that includes a first type of low-k material. of the lowersegment. The inner spacers 902C are also formed from the first type oflow-k material. In embodiments of the invention, the lower segment 902Bof the two-segment outer spacers 1202 is the same material as the innerspacers 902C.

In general, a material is considered low-k when its k-value ordielectric constant is similar to or less than the k-value of SiO₂,which is generally accepted to be about 3.9. However, for gate spacerapplications formed from SiN to provide greater process control,materials with k-values lower than SiN could be considered as low-kspacer material. The k-value of SiN is about 7.0. In embodiments of theinvention, the first type of low-k material can be a dielectric materialhaving a dielectric constant in the range from about 4 to about 6.Examples of the first type of low-k material includes but is not limitedto SiBCN, SiOCN, and the like. In embodiments of the invention, thesecond type of low-k material can be a dielectric material having adielectric constant in the range from about 2 to about 6. Examples ofthe second type of low-k material includes but is not limited to SiOCN,SiOC, SiCOH, and the like, with the caveat that the second type of low-kmaterial is different than the first type of low-k material.

In FIG. 15, known semiconductor device fabrication processes have beenused to form source/drain (S/D) regions 1310, 1312. In embodiments ofthe invention, the S/D regions 1310, 1312 are formed using an epitaxiallayer growth process on the exposed sidewalls of the Si channelnanosheet regions 122A, 124A, 126A. In-situ doping (ISD) can be appliedto form doped S/D regions 1310, 1312, thereby creating the necessaryjunctions in the nanosheet transistors 140, 140A (shown in FIG. 1).Virtually all semiconductor transistors are based on the formation ofjunctions. Junctions are capable of both blocking current and allowingit to flow, depending on an applied bias. Junctions are typically formedby placing two semiconductor regions with opposite polarities intocontact with one another. The most common junction is the p-n junction,which consists of a contact between a p-type piece of silicon, rich inholes, and an n-type piece of silicon, rich in electrons. N-type andp-type FETs are formed by implanting different types of dopants toselected regions of the device to form the necessary junction(s). N-typedevices can be formed by implanting arsenic (As) or phosphorous (P), andp-type devices can be formed by implanting boron (B).

In embodiments of the invention, the epitaxial growth and in-situ dopingused to form the S/D regions 1310, 1312 are controlled in a manner thatforms extension regions 1502 having extension junctions 1504 within theSi channel nanosheet regions (e.g., Si nanosheet regions 126A, 126D).For ease of illustration, only one example of the extension regions 1502and extension junctions 1504 are provided with reference numbers.Extension regions 1502 and extension junctions 1504 extend undertwo-segmented offset gate spacers 1202, and the regions of the Sichannel nano sheets 122A, 124A, 126A under the gate dummy gates 304remain undoped. In embodiments of the invention, the doping of extensionregions 1502 can be performed by implanting the end regions of the Sichannel nanosheet regions 122A, 124A, 126A with ions (or dopants) (e.g.,through epitaxy or through ion implantation). Extension junctions 1502reduce the R_(ext) of the nanosheet transistors 140, 140A (shown inFIG. 1) when they are turned on by applying the appropriate gate biasvoltages. The extension junctions 1502 also form a conducting pathbetween the S/D regions 1310, 1312 and the Si channel nanosheet regions122A, 124A, 126A. In order to optimize transistor performance, theextension junctions 1504 can be positioned such that they terminate insubstantially the same plane that the final SG metal gate structure 1406(shown in FIG. 1) and/or the final work function metal (WFM) gatestructure 1404 (shown in FIG. 2) begin.

Referring still to FIG. 15, known semiconductor fabrication operationshave been used to form an interlayer dielectric (ILD) region 1302. TheILD region 1302 can be formed by depositing an oxide material in bulk(e.g., silicon dioxide) and polishing the bulk oxide back (e.g., usingCMP) to the level of the upper segment 1102A and the hard masks 308.

Referring back to FIG. 1, known semiconductor fabrication operationshave been used to remove the dummy gates 304 and the hard masks 306,308. In embodiments of the invention, the dummy gates 304 and the hardmasks 306, 308 can be removed by a known etching process, e.g., RIE orchemical oxide removal (COR). Additionally, known semiconductorfabrication operations have been used to remove the SiGe sacrificialnanosheet central regions 112B, 114B, 116B, 118B (shown in FIG. 15). Inembodiments of the invention, the SiGe sacrificial nanosheet centralregions 112B, 114B, 116B, 118B can be removed by applying a selectiveetch (e.g., a hydrochloric acid (HCl)).

In FIG. 1, known semiconductor fabrication processes (e.g., a gate-latefabrication process) have been used to replace the removed dummy gates304, hard masks 306, 308, gate dielectric 302B, and sacrificialnanosheets 112B, 114B, 116B, 118B with a multi-segmented metal gatestructure 1404, 1406 and a relatively thin (e.g., from about 1 nm toabout 3 nm) gate dielectric (e.g., high-k (HK)) layer 1402. Inembodiments of the invention, the relatively thin gate dielectric layer1402 can be formed from a hafnium-based material. In embodiments of theinvention, the gate metal structure 1404 can include a metal liner,e.g., a WFM 1404. In embodiments of the invention, the WFM 1404 can be,for example, TiN or TaN. In embodiments of the invention, the metal gatestructure 1406 can be a metal gate formed from, e.g., aluminum ortungsten. The multi-segmented metal gate structure 1404, 1406 and therelatively thin gate dielectric layer 1402 surround the stacked Sinanosheet channel regions 122A, 124A, 126A and regulate electron flowthrough the Si nanosheet channel regions 122A, 124A, 126A between thesource 1310 and the drain 1312. In order to optimize transistorperformance, the extension junctions 1504 can be positioned such thatthey terminate in the same plane that the final SG metal gate structure1404, 1406 begins.

The methods and resulting structures described herein can be used in thefabrication of IC chips. The resulting IC chips can be distributed bythe fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includes ICchips, ranging from toys and other low-end applications to advancedcomputer products having a display, a keyboard or other input device,and a central processor.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the detaileddescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Similarly, the term “coupled” and variations thereofdescribes having a communications path between two elements and does notimply a direct connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification. Accordingly, a coupling ofentities can refer to either a direct or an indirect coupling, and apositional relationship between entities can be a direct or indirectpositional relationship. As an example of an indirect positionalrelationship, references in the present description to forming layer “A”over layer “B” include situations in which one or more intermediatelayers (e.g., layer “C”) is between layer “A” and layer “B” as long asthe relevant characteristics and functionalities of layer “A” and layer“B” are not substantially changed by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The terms “about,” “substantially,” “approximately,” “slightly lessthan,” and variations thereof, are intended to include the degree oferror associated with measurement of the particular quantity based uponthe equipment available at the time of filing the application. Forexample, “about” can include a range of ±8% or 5%, or 2% of a givenvalue.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The term “conformal” (e.g., a conformal layer) means that the thicknessof the layer is substantially the same on all surfaces, or that thethickness variation is less than 15% of the nominal thickness of thelayer.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device and IC fabrication may or maynot be described in detail herein. By way of background, however, a moregeneral description of the semiconductor device fabrication processesthat can be utilized in implementing one or more embodiments of thepresent invention will now be provided. Although specific fabricationoperations used in implementing one or more embodiments of the presentinvention can be individually known, the described combination ofoperations and/or resulting structures of the present invention areunique. Thus, the unique combination of the operations described inconnection with the fabrication of a semiconductor device according tothe present invention utilize a variety of individually known physicaland chemical processes performed on a semiconductor (e.g., silicon)substrate, some of which are described in the immediately followingparagraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), chemical-mechanicalplanarization (CMP), and the like. Reactive ion etching (RIE), forexample, is a type of dry etching that uses chemically reactive plasmato remove a material, such as a masked pattern of semiconductormaterial, by exposing the material to a bombardment of ions thatdislodge portions of the material from the exposed surface. The plasmais typically generated under low pressure (vacuum) by an electromagneticfield. Semiconductor doping is the modification of electrical propertiesby doping, for example, transistor sources and drains, generally bydiffusion and/or by ion implantation. These doping processes arefollowed by furnace annealing or by rapid thermal annealing (RTA).Annealing serves to activate the implanted dopants. Films of bothconductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators(e.g., various forms of silicon dioxide, silicon nitride, etc.) are usedto connect and isolate transistors and their components. Selectivedoping of various regions of the semiconductor substrate allows theconductivity of the substrate to be changed with the application ofvoltage. By creating structures of these various components, millions oftransistors can be built and wired together to form the complexcircuitry of a modern microelectronic device. Semiconductor lithographyis the formation of three-dimensional relief images or patterns on thesemiconductor substrate for subsequent transfer of the pattern to thesubstrate. In semiconductor lithography, the patterns are formed by alight sensitive polymer called a photo-resist. To build the complexstructures that make up a transistor and the many wires that connect themillions of transistors of a circuit, lithography and etch patterntransfer steps are repeated multiple times. Each pattern being printedon the wafer is aligned to the previously formed patterns and slowly theconductors, insulators and selectively doped regions are built up toform the final device.

The flowchart and block diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of fabricating a semiconductor device,the method comprising: performing fabrication operations to form ananosheet field effect transistor (FET) device on a substrate, whereinthe fabrication operations include: forming gate spacers along a gateregion of the nanosheet FET device; wherein each of the gate spacerscomprises an upper segment and a lower segment; wherein the lowersegment comprises a first type of material.
 2. The method of claim 1,wherein the upper segment comprises a second type of material.
 3. Themethod of claim 2, wherein the first type of material comprises a firsttype of low-k material.
 4. The method of claim 3, wherein the secondtype of material comprises a second type of low-k material.
 5. Themethod of claim 2, wherein the fabrication operations further compriseforming inner spacers between channel nano sheets of the nanosheet FETdevice.
 6. The method of claim 5, wherein inner spacers comprise thefirst type of material.
 7. The method of claim 6, wherein the first typeof material comprises a first type of low-k material.
 8. The method ofclaim 7, wherein the second type of material comprises a second type oflow-k material.
 9. The method of claim 1, wherein forming the gatespacers comprises: forming dummy gate spacers along the gate region ofthe nanosheet FET device; using the dummy gates spacers as masks toremove portions of channel nanosheets of the nanosheet FET device; andreplacing the dummy gate spacers with the gate spacers.
 10. The methodof claim 9, wherein replacing the dummy gate spacers with the gatespacers comprises: forming a layer of the first type of material alongthe gate region of the nanosheet FET device; and replacing an upperregion of the layer of the first type of material with a layer of thesecond type of material; wherein the layer of the second type ofmaterial comprises the upper segment of the gate spacer.
 11. The methodof claim 10, wherein replacing the dummy gate spacers with the gatespacers further comprises forming the lower segment of the gate spacerby using the upper segment of the gate spacer as a mask for removingportions of a lower region of the layer of the first type of material.12. A method of fabricating a semiconductor device, the methodcomprising: performing fabrication operations to form a nanosheet fieldeffect transistor (FET) device on a substrate, wherein the fabricationoperations include: forming gate spacers along a gate region of thenanosheet FET device; and forming inner spacers between channelnanosheets of the nanosheet FET device; wherein each of the gate spacerscomprises an upper segment and a lower segment; wherein the uppersegment comprises a second type of material; wherein the lower segmentcomprises a first type of material; wherein inner spacers comprise thefirst type of material; wherein the first type of material comprises afirst type of low-k material.
 13. The method of claim 12, wherein thesecond type of material comprises a second type of low-k material. 14.The method of claim 12, wherein forming the gate spacers comprisesforming dummy gate spacers along the gate region of the nanosheet FETdevice.
 15. The method of claim 14, wherein forming the gate spacersfurther comprises using the dummy gates spacers as masks to removeportions of channel nanosheets of the nanosheet FET device.
 16. Themethod of claim 15, wherein forming the gate spacers comprises replacingthe dummy gate spacers with the gate spacers.
 17. The method of claim16, wherein replacing the dummy gate spacers with the gate spacerscomprises forming a layer of the first type of material along the gateregion of the nanosheet FET device.
 18. The method of claim 17, whereinreplacing the dummy gate spacers with the gate spacers further comprisesreplacing an upper region of the layer of the first type of materialwith a layer of the second type of material.
 19. The method of claim 18,wherein the layer of the second type of material comprises the uppersegment of the gate spacer.
 20. The method of claim 19, whereinreplacing the dummy gate spacers with the gate spacers further comprisesforming the lower segment of the gate spacer by using the upper segmentof the gate spacer as a mask for removing portions of a lower region ofthe layer of the first type of material.